Induction motor control system



May 28, 1940.

R. A. GEUDER INDUCTION MOTOR Filed Nov.

CONTROL SYSTEM /CPGZ Patented May 28, 1940 UNITED STATES PATENT OFFICERichard A. Gender, Cleveland, Ohio, assigner to Reliance Electric andEngineering Company, a

corporation of Ohio Application November 10, 1938, Serial No. 239,809

5 Claims.

My invention relates, in general, to motor control systems, and moreparticularly to induction motor control systems for transferringmaterial from either of two rotatively mounted members to the other.

My invention is diagrammatically illustrated in combination with a pairof reels on either side of a processing device through which thematerial being transferred from one reel to the other must pass.Although my invention is capable of general application, it isparticularly useful for transferring a coiled strip of steel from onereel to another while passing same through a processing device to polishthe surface of the steel. When the coiled strip of steel is passedthrough the processing device in one direction, the direction of travelis reversed and then it is rewound upon the original reel. This cycle ofoperation is continued until the steel strip is properly finished.

An object of my invention is to provide for transferring material fromeither of two reels to the other at a substantially constant speed andtension, as the eifective diameters of the reels vary.

Another object of my invention is to provide for reversibly operating apair of induction motors as a motor and a drag generator, and as a draggenerator and a motor, respectively, and for causing the drag generatorto produce a brakingtorque characteristic which varies approximatelyinversely with the speed of the unwinding reel to maintain substantiallyuniform tension on the material being unwound and for causing theinduction motor when acting as a motor to decrease its speed as thedriving torque increases resulting from effective increase in thediameter `of the Winding reel.

Another object of my invention is to provide for reversely operating aninduction motor as a motor and as a drag generator, and for causing theinduction motor when acting as a motor to decrease its speed as thedriving torque increases resulting from an effective increase in thediameter of the winding reel and for causing the induction motor whenacting as a drag generator to decrease its braking torque as the speedof the unwinding reel increases resulting from an effective decrease linthe diameter of the unwinding reel.

Another object of my invention is to provide for operating an inductionmotor as a drag generator and for producing a braking-torquecharacteristic which varies approximately inversely with the speed ofthe drag generator.

Another object of my invention is to provide for balancing the directcurrent excitation of the stator of an induction motor with theselectionV of the rotor resistance to produce a brakingtorquecharacteristic which compensates for the eifective decreasing diameterof an unwinding reel.

Other objects and a fuller understanding of my invention may be had byreferring to the following description and claims, taken in conjunctionWith the accompanying drawing, in which:

Figure 1 is a diagrammatic illustration of a control system embodyingthe features of my invention; and

Figure 2 is a group of curves showing the performance of my controlsystem.

With reference to the drawing, my control system comprises, in general,an induction motor I0 having a stator Il and a rotor I2 suitablyconnected to a reel I3 by means of a shaft I4, an induction motor Ilhaving a stator I8 and a rotor I9 suitably connected to a reel 2E) bymeans of a shaft 2I, a plurality of groups of resistance units 2E, abank of adjustable resistance units 2l, a segmental controller 28, and amotor-generator set 24.

In the embodiment of my invention, the induction motors I0 and I 'I arearranged to be reversibly operated as a motor and as a drag generator,and as a drag generator and as a motor, respectively, to transfer amaterial 22 from either one of the two reels I3 and 26 to the other. Thematerial 22 as it is transferred from either of the two reels to theother is passed through a processing device 23 for finishing thematerial. 'I'he processing device 23 may be one of any number ofdevices, and may comprise a device for finishing the surface of a coiledstrip of steel as the steel passes therethrough. In actual practice, theprocessing device may comprise a series of surfacing pads having sandpaper or other suitable material arranged to polish the coiled strip asit is transferred from either of the two reels to the other. While Ihave described my invention briefly in connection with a processingdevice for finishing the surface of a steel strip, it is to beunderstood that my control system applies equally well to other types ofprocessing devices.

When the reel I3 is acting as a winding reel and the reel 20 is actingas an unwinding reel, the induction motor I0 is operating as a motor andthe induction motor I'I is operating as a drag generator. When theinduction motor IG is operating as a motor and the induction motor Il isoperating as a drag generator, the effective diameter of the reel I3 isgradually increasing and the effective diameter of the reel 20 isgradually decreasing. In the practice of my invention under the aboveassumed condition, provide for governing the induction motor l! to givean increasing torque which compensates for the effective increase in thediameter of the winding reel, and thereby cause the material 22 totravel at a substantially constant transfer speed. At the same time, theinduction motor il, which is acting as a drag generator, is operated togive a brakingtorque characteristic which varies substantially inaccordance with the effective decreasing diameter of the unwinding reel20.

In order to increase the torque of the inductance motor Iii and at thesame time decrease the speed thereof to compensate for the eiectiveincrease in diameter of the winding reel it., I arr-ange for connectingthe rotor i2 of the induction motor It, when acting as a motor, incircuit relation with the plurality of groups of resistance units Zit.The plurality of groups of resistance units E@ are a1'- l'anged to beprogressively inserted step-by-step in the rotor circuit by means of thesegmental controller 2t.

The induction motor il', when acting as a drag generator, is arranged togive a braking torque characteristic which varies approximatelyinversely with the increase in speed of the unwinding reel Zii. Toaccomplish this, I arrange for exciting the stator it by direct currentfrom the conductors 313 and 35 supplied by the generator ti of themotor-generator set fill, and for connecting the rotor i9 of theinduction motor ill with the adjustable bank oi resistance units 2i. Thecharacteristic of the braking torque of the induction motor il, whenacting as a drag generator, is determined by balancing the directcurrent excitation of the stator with the selection or the proper amountof resistance 2li. With reference to Figure 2, a proper balance of thedirect current excitation of the stator with the adjustable resistanceof the rotor will produce a speed-v torque curve it@ which substantiallyparallels the curve lili which represents the ever-decreasing effectivediameter of the unwinding reel with respect to the speed of the reel.The vertical distance between the two curves iiii and itl reppresentsthe value oi the drag upon the material 22 as it passes through theprocessing device 23. The direct current excitation or" the stator maybe varied by adjusting the field rheostat which governs the eld strengthoi the field winding 32 of the generator Si of the motor generator set2a. The value oi the resistance connected in circuit relation with therotor oi the induction motor il, when acting as a drag generator, may beaccomplished by adjusting the setting oi the adjustable bank ofresistance units 2l; When the proper balance between the direct currentexcitation and the resistance 2l is determined,

then the adjustable rheostat 33 and the bank of.

resistance units 2l are fixed.

In explaining the operation oi my invention, let it be assumed that theswitch 353 is closed which energizes the motor 3@ or the motor-generator2t from the pair of conductors @it and lil connected across the powerconductors 3G and ti, and that the control arm i5@ of the segmentalcontroller 28 is in its vertical position as shown in the drawing. 1nstarting my control system, let it be further assumed that it is desiredto operate the induction motor iii as a motor and the induction motor Ilas a drag generator. Under this assumed condition, the operator quicklyactuates the control arm it@ to the right and causes the arcuatebridging segment @l to bridge all of the respective contact segments 62,63,

fili, 55, 65 and al'. Just as soon as the arcuate bridging segment 5lbridges all of the contact segments Si to 6l, inclusive, there are aplurality of circuits established for causing the induction motor iii tooperate as a motor and the induction motor il to operate as a draggenerator. The establishment of the respective plurality of circuits maybe described as follows:

Starting with the connection l@ on the alternating current conductor to,current iiows through the conductor 'iii and the control arm to thearcuate bridging segment 6l, and thence through the respective segmentalcontacts GZ to 6l, inclusive. The current upon leaving the segmentalcontact 52 iiows through a common return conductor Si? which continuesto the connection il on the alternating current conductor il. Theclosing or" the group of three contactors 3 impresses three phaseexcitation upon the stator il of the induction motor i from the threepower conductors 36, 3l and 38. The current upon leaving the segmentalcontact 52 also flows through a conductor il to energize the coil oi thecontactor iii and from there the current flows through the common returnconductor St to the connection il. The closing of the group oi threecontactors it connects the rotor i2 of the induction motor iii incircuit relation with the plurality of groups of resistance units t6.rl'he current upon leaving the segmental contacts likewise flows througha conductor 32 which branches ofi through the conductors it and toenergize respectively the coils oi the contactors c5 and @5, and fromthese two coils the current iiov/s through the com mon return conductorSt to the connection il. The closing of the group of two contactors :i5impresses direct current excitation upon the stator winding of theinduction motor il yacting as a drag generator. The closing of the groupof three contactors it@ connects the rotor of the induction motor il',acting as a drag generator, in circuit connection with the bank ofresistance units 2i.'

The segmental contacts t3 and El, inclusive, and the segmental contactst@ and 'i3 inclusive, are connected together as illustrated in thedrawing, and thus when the control arm 6@ is actuated to the right, thecurrent upon leaving the contact segments @53 flow through the contactsegment 259 and thence through a conductor for energizing the coil ofthe contacter 56 and from there the current flows through the comrnonreturn conductor Sii to the connection ll. The current upon leaving thesegmental contact Gt iiows through the segmental contacts lil andaconductor (it to energize the coil or the contacter 55 and from therethe current iiows through the common return conductor 8B to theconnection lli. The current upon leaving the segmental contact G35 iiowsthrough the segmental contact lll and a conductor tl to energize thecoil or the contactor mi and from there the current iiows through thecommon return conductor 8G to-the connection lil. The current uponleaving the segmental contact @c flows through the segmental contact l2and a conductor tit for energizing the coil oi the contactor 53 and fromthere the current iiows through the common return conductor Sii to theconnection il. The current upon leaving the segmental contact tl' flowsthrough the segmental contact 'It and a conductor S9 for energizing thecoil of the contacter 52 and from there the current ows through thecommon return conductor to the connection TI. Therefore, when thecontrol arm 60 is actuated to the right, causing the arcuate bridgingsegment 6| to bridge all of the segmental contacts 62 to 61, inclusive,the induction motor I0 is operating with all of the group of resistanceunits 26 excluded from the rotor circuit I2, since all of the contactors52 to 56 are closed.

The speed-torque characteristic of the induction motor I0, operatingwith all of the resistance units 26 excluded is represented by the curveI 02 in Figure 2, and the speed of the motor when all the group ofresistances 26 are excluded is determined by the intersection of thecurve |02 with the curve |0|. Let it be assumed for the purpose ofsimplicity that when the reel I3 is empty the effective diameter isone-half of what it will be when the reel I3 is full, which means thatthe value of the torque required to rotate the empty reel is one-half ofthat required to operate the full reel. Thus, in Figure 2, the curve |0|also represents the increasing driving torque required to drive the reelI3 as it increases in effective diameter, as the speed decreases.Therefore, in order to provide for an increase in the torque and areduction in the speed of the induction motor I0, when acting as amotor, I provide for inserting step-by-step the several groups ofresistance units 26 in the rotor circuit of the induction motor I0. Inactual practice, when the operator observes, after the motor I0 has beenoperating for a period of time, that the travel of the material 22increases slightly above its normal transfer speed, he actuates thecontrol arm 60 to the left to unbrdge the contact segments 61 whichcauses the contactor 52 to open and insert the rst banks of resistanceunits 26 in the rotor circuit I2 of the induction motor I0. Thespeed-torque characteristics of the induction motor I0 with the rstbanks of resistance units 26 in the rotor circuit may be designated bythe curve |03. The introduction of the rst bank of resistance units 26in the rotor circuit I2 causes the speed to reduce at a value determinedwhere the curve |03 intercepts the curve 6| The operator allows themotor to operate under this condition for a period of time until thetravel of material 22 increases slightly beyond its normal Value andthen he de-notches to the left to unbridge the segmental contact 66,which opens the contactor 53 and inserts ythe first two banks ofresistance units 26 in the rotor circuit I2. The speed-torquecharacteristic of the induction motor I0 with two banks of resistanceunits 26 inserted into the rotor circuit I2 may be represented by thecurve |04. Therefore, the speed of the induction motor I0 is decreasedto an amount determined where the curve |04 intercepts the curve |0|.The operator continues to de-notch the arcuate bridging segment 6| tothe left until all of the contact segments 6l to 63 are unbridged,step-by-step, introducing the group of resistance units 26 into therotor circuit I2 of the induction motor I0, producing respectively thetorque-speed characteristic curves |05, |06, and |07. The speed of theinduction motor accordingly is decreased, step-by-step to valuesdetermined by the intersection of the curves |05, |06 and |01 with thecurve |0I.

The tension of the material 22 is maintained substantially constant, asdetermined by the braking-torque characteristic of the induction motorIl, acting as a drag generator, which is substantially inverselyproportional to the speed of the reel 20.

When the material 22 is transferred to the reel I3, the process isreversed and the induction motor Il is operated as a motor, and theinduction motor I0 is operated as a drag generator. To take care of thisreverse operation, the segmental controller 28 is actuated to the left,in which case the sequence of the control is the same as that describedwith reference to the control when the control arm 60 is actuated to theright. When the arcuate bridging segment 6| bridges all of the contacts68 to 13, inclusive, the current upon leaving the contact 68 flowsthrough a conductor 80 which branches into the two conductors 6| and 92for energizing respectively the two coils of a contactor 48 and 47 andfrom there the current iiows through the common return conductor 80 tothe connection '11. The closing of the group of three contactors 41impresses three phase excitation upon the stator of the induction motor|'I from the supply conductors 36, 31 and 38. The closing of the groupof three contactors 48 connects the rotor |9 of induction motor I'I incircuit relation with the bank of resistance unit 26. The current uponleaving the segmental contact 68 also fiows through a conductor 93 whichbranches off into the conductor 96 and 95 to energize respectively thecoils of the contactors 46 and 50 and from there the current flowsthrough the common return conductor 80 to the connection l1. The closingof the group of three contactors 49 connects the bank of resistanceunits 2l in circuit relation with the rotor I2 of the induction motor I0and the closing of the groupof two contactors 50 impresses directcurrent excitation upon stator windings of the induction motor I0. Thedenotching of the control arm 60 to the right to insert step-by-step thesuccessive banks of resistance units 26 in the rotor circuit of theinduction motor is the same as that previously described, When the motorI0 was acting as a motor and the induction motor I1 was acting as a draggenerator.

Therefore, from the above description, it is observed that I haveprovided for reversibly operating a pair of induction motors as a motorand as a drag generator, and as a drag generator and a motor,respectively, and for causing the drag generator to produce abrake-torque characteristic which varies approximately inversely withthe speed of the unwinding reel to maintain a substantially uniformtension upon the material being unwound for causing the induction motorwhen acting as a motor to decrease its speed as the driving torqueincreases resulting from an increase in the effective diameter of thewinding reel. Y

In the practice of my invention, the induction motors may be of theslip-ring induction type or of the squirrel-cage induction type. In thepresent application, I have shown and described my invention withreference to the slip-ring type, but a squirrel-cage type may beemployed by selecting the proper value of xed resistance in thesquirrel-cage rotor windings and balancing the selected value with thedirect current excitation to produce the required braking-torquecharacteristic.

While I have shown a motor-generator set as the means for providing thedirect current excitation, it is understood that my invention includesother means of providing direct current excitation, such, for example,as a rectifier arrangement, a battery; or a condenser across two of thethree terminals oi the polyphase induction motor acting as the draggenerator.

Although I have described my invention with a certain degree ofparticularity, it is understood that the present disclosure has beenmade only by way of example and that numerous changes in the details ofconstruction and the combination and arrangement of parts may beresorted to without departinfj trom the spirit and the scope of theinvention as hereinafter claimed.

I claim as my invention:

l. In combination with a source ci direct current and of alternatingcurrent, of a control system for unwinding material from either of tworotatively mounted rolls and winding same upon the other comprising, afirst induction motor having a stator and a rotor connected to one ofsaid rolls, a second induction motor having a stator and a rotorconnected 'to the other said roll, a first resistance means, secondresistance means, controlling means comprising irst control means foroperating the iirst induction motor as a motor and the second inductionmotor as a drag generator to unwind the material from one of said rollsand winding same upon the other roll and second control means foroperating the second induction motor as a motor and the irst inductionmotor as a drag generator to unwind the material from the latter of saidrolls and wind same upon the former roll, said first control meanscomprising a plurality of switch means, one said switch means connectingthe stator oi' the first induction otor to thc source of alternatingcurrent, another said switch means connecting the rotor ci the iirstinduction motor to the l'irst resistance means, another said switchmeans connecting the stator oi the second induction motor to the or"direc current and another ci said switch means connecting the rotor ofthe second induction motor to the second resistance means, control meanscomprising a plurality of switch means, one said switch means connectingthe er of the second induction motor to the source or alternatingourrent, another said switch means connecting the rotor oi the secondinduction motor to the :first resistance means, another switch meansconnecting the stator oi the lirst induction motor to the source ofdirect current and another said switch means connectAA the rotor of therst induction motor to the second resistance means, means for selectingthe direct current excitation oi the stator of the induction motoracting as a drag gen *tor with the value ci the second resistance meansto cause induction motor actas a drag generator to produce a brakingtorque characteristic which varies approximately inversely with thespeed of the unwinding roll to maintain substantially uniform tension onthe mate iai being unwound, and means for varying the value ci the r'ist resistance means to reduce the speed oi the induction motor acting asa motor to transfer the material to the winding roll at substantiallyconstant lineal speed.

2. In combination with a source oi direct current and or alternatingtrent, oi control system for unwinding rrateilal from an unwinding rolland winding same upon a winding roll comprising, a First induction motorhaving a stator and a rotor connected to the winding roll, a secondinduction motor having stator and a rotor connected to the unwindingroll, a rlrst resistance means, a second resistance means, controllingmeans for operating the ilrst induction motor as a motor and the secondinduction motor as a drag generator to unwind the material from theunwinding roll and wind same upon the winding roll, said controllingmeans comprising a plurality of switch means, one said switch meansconnecting the stator of the first induction motor to the source ofalternating current, another said switch means connecting the rotor ofthe rst induction motor to the first resistance means, another saidswitch means connecting the stator of the second induction motor to thesource of direct current and another of said switch means connecting therotor of the second induction motor to the second resistance means,means for selecting the direct current excitation of the stator of theinduction motor acting as a drag generator with the value of the` secondresistance means to cause the induction motor acting as a drag generator'to produce a braking torque characteristic which Varies approximatelyinversely with the speed of the unwinding roll to maintain substantiallyuniform tension on the material being unwound, and means for varying thevalue of the first resistance means to reduce the speed of the inductionmotor acting as a motor to transfer the material to the winding roll ata substantially constant lineal speed.

3. The method oi' unwinding material from an unwinding roll and windingsame upon a winding roll at a substantially constant transfer linealspeed and at a substantially constant tension comprising, riving thewinding roll 'by an induction motor having a stator and a rotor circuit,exciting the stator oi the induction motor by alternating current andestablishing a minimum of resistance in the rotor circuit, increasinglyintroducing resistance in the rotor circuit and reducing the speed ofthe winding roll substantially in accordance with the increasingeffective diameter of the winding roll to transfer the material atsubstantially a constant transfer lineal speed, opposing the rotation ofthe unwinding roll by an induction motor having a stator and a rotorwith resistance, exciting the stator of the second induction motor bydirect current to cause the second induction motor to act as a draggenerator, selecting the direct current excitation with the value of therotor resistance of the second induction motor to produce a brakingtorque characteristic which varies approximately inversely with theincreasing speed oi the unwinding roll to maintain a substantiallyuniform tension on the material being unwound.

4. The method of unwinding material from an unwinding roll and windingsame upon a winding roll at a substantially constant transfer linealspeed and at a substantially constant tension comprising', driving thewinding roll by an induction motor having a stator and a rotor circuit,exciting the stator of the induction motor by alterhating current andestablishing a minimum of resistance in the rotor circuit, increasinglyintroducing resistance in the rotor circuit and reducing the speed ofthe winding roll substantially in accordance with the increasingeiective diameter of the winding roll to transfer the material atsubstantially a constant transfer lineal speed, opposing the rotation ofthe unwinding roll by an induction motor having a stator and a rotorwith resistance, exciting the stator oi' the second induction motor 'bydirect current to cause the second induction motor to act as a draggenerator, selecting the direct current excitation with the Value of therotor resistance of the second induction motor to produce a brakingspeedof the unwinding roll by an induction motor having a stator and arotor with resistance, exciting the stator of the induction motor bydirect current to canse the induction motor to act as a drag generator,selecting the direct current excitation 5 with the value of the rotorresistance of the induction motor toy produce a braking torquecharacteristic which varies approximately inversely with the increasingspeed of the unwinding roll to maintain a substantially uniform tensionon the 10 material being unwound.

RICHARD A. GEUDER.

